This is entirely doable using Nintex Forms and some simple validation rules. I provided an export of what I did so that you can follow along if needed.

First off, I created this on a list with the following columns:

Title - not required

Choice (Choice field)

Dropdown Required

Textbox Required

Dropdown (Choice Field)

A

B

C

Textbox (Single Line Text)

The form will require that a selection is made from Dropdown when "Dropdown Required" is selected from Choice. Conversely, it will require that something be in Textbox if "Textbox Required" is selected from Choice.

I dropped the Dropdown and Textbox controls into panels so that it is easier to manage the rules that are applied to them. On the Dropdown panel, I placed the following rule:

This will Hide the panel if "Dropdown Required" is not selected. As you can guess, setup a similar rule on the Textbox panel to do the same thing like so:

This takes care of hiding and showing the controls based on the Choice selection. Now to add in the validations.

Select the Dropdown control and add the following rule:

This will check Choice to see if it equals "Dropdown Required" AND if Dropdown is empty. If TRUE, it will cause validation and not allow the user to submit:

Now for the Textbox control. Same approach; select the control and add the following validation rule:

Again, it will check Choice to see if it equals "Textbox Required" AND if Textbox is empty. If TRUE, it will cause validation and not allow the user to submit:

From here you can drop these controls into a repeating section and alter the message that will appear.

This is entirely doable using Nintex Forms and some simple validation rules. I provided an export of what I did so that you can follow along if needed.

First off, I created this on a list with the following columns:

Title - not required

Choice (Choice field)

Dropdown Required

Textbox Required

Dropdown (Choice Field)

A

B

C

Textbox (Single Line Text)

The form will require that a selection is made from Dropdown when "Dropdown Required" is selected from Choice. Conversely, it will require that something be in Textbox if "Textbox Required" is selected from Choice.

I dropped the Dropdown and Textbox controls into panels so that it is easier to manage the rules that are applied to them. On the Dropdown panel, I placed the following rule:

This will Hide the panel if "Dropdown Required" is not selected. As you can guess, setup a similar rule on the Textbox panel to do the same thing like so:

This takes care of hiding and showing the controls based on the Choice selection. Now to add in the validations.

Select the Dropdown control and add the following rule:

This will check Choice to see if it equals "Dropdown Required" AND if Dropdown is empty. If TRUE, it will cause validation and not allow the user to submit:

Now for the Textbox control. Same approach; select the control and add the following validation rule:

Again, it will check Choice to see if it equals "Textbox Required" AND if Textbox is empty. If TRUE, it will cause validation and not allow the user to submit:

From here you can drop these controls into a repeating section and alter the message that will appear.